Where to get Augments

As I am prepping my mats for my level 50 craftables, I am unclear where to get higher level augments. More specifically, Thick or Shimmering Naga Scale. Do these only drop from level 50 rifts or invasions? Are they farmable anywhere?

I kinda like the system. Gives planarite some value in the endgame. The way I approach it if I want a particular aug is to decide how much planarite i'm willing to dump and then buy the chests. I hang on to the ones I like and AH the rest. From there you just need to monitor the AH over time and snatch up the augs you want when you see them at a good price. A little frustrating when you are impatient like me but you should only really be going through the hastle for gear that you intend to wear for a long time. Planarite caps out at 30k so I expect that over time as people try to cash in their planarite that we'll see the availability of augs go up and the price go down.